System and method for detecting water vapor within natural gas

ABSTRACT

A system and method are disclosed for the detection of water vapor in a natural gas background. The system includes a light source operating in a wavelength range such as, 1.877-1.901 μm, 2.711-2.786 μm, or 920-960 nm, passes through the natural gas to be detected by a detector. In one embodiment, the light source is a tunable diode laser and the moisture level is determined by harmonic spectroscopy. In other embodiments, a VCSEL laser is utilized.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation of U.S. patent applicationSer. No. 10/688,723, filed Oct. 16, 2003, which is aContinuation-in-Part of U.S. patent application Ser. No. 09/941,891,filed Aug. 28, 2001, which claims the benefit of U.S. Patent ApplicationSer. No. 60/228,494, of Dr. Randy D. May, filed Aug. 28, 2000, all ofwhich are hereby fully incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a system and method for the detectionof moisture in natural gas. More specifically, the present inventionrelates to a technique for determining the level of water vapor presentwithin an industrial natural gas pipeline.

Natural gas has long been used as an energy source because of its lowcost and widespread availability. After natural gas is mined, it ispurified through several sequential processes, and distributed vianetworks of underground pipelines that typically transport the gas at apipe pressure of several hundred pounds per square inch (PSI). Naturalgas is sold to the customer as an energy product, and the energy contentis generally expressed in British Thermal Units (BTU). The rate thatgaseous product is pumped to the customer is measured in standardmillion cubic feet (SMCF), which is based on the gas volume at astandard pressure and temperature (typically 1 atmosphere pressure/14.73PSI, and 70 degrees F.).

Contaminants in natural gas, such as water, reduce the BTU capacity ofthe gas, thereby resulting in a less efficient energy product.Contaminants also corrode delivery pipelines over time potentiallyresulting in serious safety hazards while also necessitating the costlyreplacement of segments of the pipeline (downtime for the pipelines cancost upwards of several thousand dollars per second). Accordingly,companies engaged in the mining, purification, and distribution ofnatural gas continuously monitor the quality of the gas at variousstages of production and distribution to prevent such occurrences. Onecontaminant of particular interest is water vapor (H₂O). Excessivebuildup of water vapor is a primary cause of pipeline corrosion, and itacts to dilute the natural gas thereby reducing its BTU capacity(thereby making the gas a less efficient energy source).

Distributors of natural gas typically have set maximum allowable levelsof H₂O within natural gas for various stages of natural gas productionand distribution. The final product that is delivered to the customer(usually a large consumer supplier such as Southern California Gas, orPacific Gas and Electric), is termed “mainline gas.” The typical maximumallowable level of H₂O in mainline gas is 7 lbs of H₂O per measuredmillion standard cubic feet of CH₄ (MMscf); 1 lb/MMscf is approximately21.1 parts per million by volume, ppmv). This level is termed the“tariff”. When H₂O levels exceed tariff levels, plant operation can besuspended resulting in substantial loss of revenue and associatedcustomer lawsuits.

Conventional techniques for measuring water vapor in natural gas relyprimarily on the use of chemical sensors. These sensors operate bymonitoring the capacitance or dielectric constant of a sensor element(made from compounds such as phosphorous pentoxide (P₂0₅) and aluminumoxide) subjected to a sample from the mainline gas. The electricalproperties of the sensors change in a quantitative measurable manner asa function of the amount of water vapor present in the sample gas andsuch changes are translated into water concentration measurements. Insuch chemical sensors, a low pressure sample of pipeline gas isdelivered to the sensor element via a regulation (pressure reduction)system. The gas sample measured by the pipeline is at a much lowerpressure than the pipeline itself (typically 10-30 PSI, compared to 800PSI in the pipeline). Such sensors are typically housed in samplingshelters that also house the accompanying regulation system.

As the sensing elements in chemical sensors are necessarily exposed togas samples, contaminants in the gas stream such as glycols, amines, andoils directly contact the sensors. While chemical sensors can providereliable measurements for short periods of time after calibration, theexposure to the contaminants (glycols and amines in particular) soil thesensor, thereby causing drifts in the calibration. This conditionresults in erroneous readings and can lead to eventual failure if thecontaminants build up. Various filters (coalescing, adsorbents, andparticle filters) have been employed to minimize the effects of glycoland amine contamination, but historically these filtration schemes areonly temporary solutions. This is due in part because the filters areeasily saturated with contaminants or they leak and require replacementat irregular intervals.

It should therefore be appreciated that there remains a need for areliable and durable system and method for detection of water levels innatural gas.

SUMMARY OF THE INVENTION

The current invention utilizes absorption spectroscopy, a technique thathas long been utilized to measure the concentration of water vapor inair, and in various laboratory environments. With such spectroscopytechniques, a light source is passed through a gas sample and detectedby a detector opposite the light source. The light source can be aconventional hot filament, a glow bar, a laser, or any suitable emitterin the wavelength region of interest. By monitoring the amount of lightabsorbed by the sample, at specific wavelengths, the concentration ofthe target gas can be accurately determined.

A common problem with absorption spectroscopy is interference amongconstituents in the gas sample being measured. This interference occurswhen the gas of interest (in this case H₂O) absorbs light at the same,or nearly the same, wavelength as another gas present in the sample.Natural gas, which is composed of greater than 95% CH₄, has water vaporat typically less than 1% by volume. Conventional spectroscopic methods(i.e., non-laser based) are not suitable for measurements of H₂O in aCH₄ background because the absorption by CH₄, which is present in muchlarger quantities, completely obscures the much weaker absorption by H₂Oat all wavelengths in the visible and infrared region.

The current invention operates in a wavelength range with minimal CH₄absorption and preferably utilizes laser light sources for absorptionspectroscopy, thereby minimizing the effects of interference due to theextremely high spectral purity of the laser (narrow line width). In someembodiments, the current system incorporates a laser as its light sourcesuch as those used in automated, unattended, field instrumentation thatoperate at wavelengths between 1.6 and 2.7 microns (μm). The preferredlasers are the tunable diode lasers (“TDL”) detailed in U.S. Pat. No.5,257,256, which is hereby fully incorporated by reference. TDLs arewidely utilized in optical communications, laser printers, bar codereaders, CD players, and laser pointers. Alternatively, a color centerlaser which operates in the 1-3 μm region may be utilized, but suchlasers are not always suitable for use in commercial fieldinstrumentation due to their relatively large physical size, high powerconsumption, high maintenance requirements (they must be cryogenicallycooled), and cost. In addition, other types of light sources may be usedsuch as VCSELs, quantum cascade lasers, color center lasers, thatoperate at wavelengths that emit light at substantially a singlewavelength where water is absorbed at a much greater level than naturalgas, such as 920 nm to 960 nm, 1.877-1.901 μm or 2.711-2.786 μm.

Laser-based measurements of water vapor in air usecommercially-available TDLs operating at wavelengths near 1.38 μm, wherewater vapor has a strong absorption band. However, this wavelength isnot suitable for measurements of H₂O in a CH₄ background because CH₄absorption in the 1.38 micron region is extremely strong and completelyobscures absorption by H₂O (see the spectrum of CH₄ in the 1-2 μm region200 which is shown in FIG. 2).

The present system measures water vapor at another absorption band, 1.88μm, where absorption by CH₄ is much weaker (see FIG. 3 which illustratestransmission spectra 300 (transmission=1−absorption) of CH₄ 325 and H₂O350 over wavenumbers 5260-5330 (wavenumber=1/μm, times 10,000)). Thereare several H₂O absorption lines that can be used to monitor H₂O in anatural gas background, but it is within certain wavelength ranges inthe CH₄ absorption spectrum, 920 nm to 960 nm, 1.877-1.901 μm or2.711-2.786 μm, where there are relatively strong H₂O absorption lines,thereby allowing water vapor to be measured in a pure CH₄ background(see FIG. 4 which illustrates a spectrum 400 showing the relativepositions of the CH₄ 425 and H₂O 450 absorption lines over wavenumbers5322-5336). FIG. 6 illustrates a spectrum 600 showing the relativepositions of the CH₄ 625 and H₂O 650 absorption lines over wavelengths2700 nm to 2800 nm—with exemplary absorption lines at 2771.15 nm,2724.17 nm, 2740.17 nm, 2755.07 nm, 2770.69 nm and 2786.51 nm). FIG. 7illustrates a spectrum 700 showing the relative positions of the CH₄ 625and H₂0 650 absorption lines over wavelengths 920 nm to 980 nm withseveral present absorption lines.

To improve detection sensitivity, the current system employs a techniquecalled harmonic spectroscopy in connection with its TDL light source.Harmonic spectroscopy has been used since the 1950s in nuclear magneticresonance spectrometers, stark spectrometers, and other variouslaboratory instruments. Harmonic spectroscopy as used in someembodiments of the current system involves the modulation of the TDLlaser wavelength at a high frequency (kHz-MHz) and detecting the signalat a multiple of the modulation frequency. If detection is performed attwice the modulation, the term second harmonic spectroscopy is used.Advantages to this technique include the minimization of 1/f noise, andthe removal of the sloping baseline that is present on TDL spectra (dueto the fact that the laser output power increases as the laser injectioncurrent increases, and changing the laser injection current is how thelaser is tuned).

Specifically, the present invention is embodied in a system fordetecting water vapor in natural gas which includes a light sourceoperating at a wavelength at which water is absorbed at a sufficientlygreater level than natural gas, emits light through the natural gas to adetector configured to receive light from the light source, andelectronics coupled to the detector for computing the level of watervapor in the natural gas based on the amount of light detected by thedetector. In some embodiments, the light source is a tunable diodelaser, while in other embodiments, the light source is a color centerlaser or a quantum cascade laser. Furthermore, the detector ispreferably an InGaAs detector. In some embodiments, the emitted lighthas a frequency in the 1.877-1.901 μm, and in other embodiments, thelight is emitted within the range of 920 to 960 nm or 2.711-2.786 μm.Other absorption lines may be utilized where water absorbs light at asufficiently greater level than natural gas and a light source isavailable with a sufficiently small line width to emit at or near asingle absorption line.

The present invention is also embodied in a method for determining thelevel of water vapor in natural gas comprising the following steps:providing a light source emitting light at a frequency where water isabsorbed at a sufficiently greater level than natural gas, positioning adetector opposite the light source to detect the level of emitted light,supplying a sample of natural gas between the light source and thedetector, and determining the concentration of water vapor in thenatural gas based on the level of light detected by the detector. Insuch a method, the gas is preferably taken by a gas line from a mainpipeline into a shelter where the light source is housed. In somearrangements, the emitted light has a frequency approximately in the1.8-1.9 μm, more specifically, 1.877-1.901 μm, and in other variations,the light is emitted approximately at a frequency in the range of2.7-2.8 μm, and more specifically in approximate range of 2.711-2.786μm, and in yet other embodiments the light is emitted at a wavelengthwithin the range of 920 nm to 960 nm.

Though the current system is described in connection with the samplingof natural gas from a main pipeline, it will be appreciated that thecurrent system and method could be applied to any situation where it isdesirable to measure the moisture content in natural gas or methane suchas natural gas purification processes.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional sampling shelter employingchemical sensors for the detection of contaminants in methane;

FIG. 2 is a spectrum of methane at wavelengths ranging from 1.0 μm to2.0 μm;

FIG. 3 is a spectrum of methane overlaid with a spectrum of water atwavenumbers ranging from 5260 to 5330;

FIG. 4 is a spectrum of methane overlaid with a spectrum of water atwavenumbers ranging from 5322 to 5336;

FIG. 5 is a cross-sectional view of the current invention for opticallydetecting water vapor within natural gas;

FIG. 6 is a spectrum of methane overlaid with a spectrum of water atwavelengths ranging from 2700 to 2800 nm; and

FIG. 7 is a spectrum of methane overlaid with a spectrum of water atwavelengths ranging from 930 to 980 nm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The current system and method relate to the measurement of moisturecontent in natural gas based on absorption of light at specificwavelengths where water molecules absorbs light strongly. Generally,this technique is referred to as absorption spectroscopy, and isapplicable to the measurement of a wide range of gases, liquids, andsolids.

As seen in FIG. 1, a pipeline 3 of natural gas is coupled to a gas line7 which includes a regulator 11 for reducing the gas pressure within thegas line. From the regulator, the gas line enters a sampling shelter 15that houses a plurality of sensors 19 (with at least one being anoptical gas sensor as the present invention may be utilized in parallelwith the chemical sensors described above). If multiple sensors areemployed, they are connected in parallel to the gas line so that gasflow can be simultaneously directed to all of the sensors. This isaccomplished after the gas line enters the sampling shelter by divertinggas into a plurality of feed lines 27 at juncture 23. Each of the feedlines are in turn coupled to a sensor and are controlled by a valve 31to further restrict the flow of natural gas. Preferably, the gas lineand the feed lines are made from stainless steel and have outerdiameters of 0.25 inches.

As seen in FIG. 5, a gas sensor 500 which is incorporated into thesampling shelter 15, includes an inlet 503, an outlet 507, and a lightchamber 511, all of which are affixed within an optical gas sensorcasing (not shown) through a series of support flanges 517. The casingis configured to house a laser light source 519, an InGaAs detector 523adjacent to the light source 519, a window coupling the laser lightsource and the detector to the light chamber, a mirror 527 opposite thelaser light source 519, and processing electronics 531. The mirror ispositioned preferably in such a manner to reflect light emitted from thelight source through the light chamber and the window onto the detector.In one embodiment, the light source is positioned at 5 degrees fromhorizontal and the mirror is 40 cm from the light source. Preferably,the laser light source is a tunable diode laser or a VCSEL laserconfigured to emit light either in the 1.877-1.901 μm wavelength rangeor within the ranges of 920 nm-960 nm or 2.711-2.786 μm. In oneembodiment, the processing electronics includes a 16-bit Motorolamicrocontroller to convert the signals received by the detector into lbsper measured million cubic feet of methane (1 lb water/mmscf=21 ppm).

In operation, natural gas is fed into the inlet 503 of the gas sensor500 to continually pass through the light chamber until it exits the gassensor at the outlet 507. Thereafter, the processing electronics 531 areconfigured to translate the amount of light absorbed by the natural gassample into water concentration using known techniques such as thosedescribed in article by Randy D. May et al. entitled “Processing andCalibration Unit for Tunable Diode Laser Harmonic Spectrometers”, J.Quant. Spectrosc. Radiat. Transfer 49, 335-437, 1993, which is herebyincorporated by reference. Prior to coupling the gas sensor to the maingas line 7, it is preferred that a control sample of natural gas with aknown concentration of water is passed through the gas sensor forcalibration purposes.

It will be appreciated by one of ordinary skill in the art that standardtechniques such as the incorporation of a Herriott cell to replace thesingle mirror configuration described above may be utilized to increasethe effective optical path. For example, the Herriott cell couldcomprise two opposing Pyrex gold coated mirrors, each preferably with aradius of curvature of 150 mm and a diameter of 25.4 mm. In thisembodiment, the tunable diode light source, is configured within theHerriot cell so that the emitted light bounces off each mirrorapproximately 15 times. This arrangement results in an effective travelpath that is 30 times the length between the two mirrors for aneffective distance of 4 meters. The light is then detected by thedetector, which is coupled to electronics for converting the signalsreceived into water concentration measurements. It should also berecognized that depending on the application, the number of reflectionsof the Herriott cell may be adjusted. For example, if the water vaporlevels will be in the range of 5-100 lb/mmscf, then a single reflectionsystem as described above should be utilized. If the concentration levelwill be within the range 0-5 lb/mmscf, then a Herriott cell should beutilized.

It will, of course, be understood that modifications to the preferredembodiments will be apparent to those skilled in the art. For example,different techniques may be used for supplying gas samples between thelight source and the detector and for converting the signals received bythe detector into concentration measurements. Consequently, the scope ofthe present invention should not be limited by the particularembodiments discussed above, but should be defined only by the claimsset forth below and equivalents thereof.

1. A system for detecting trace amounts of water vapor in natural gascomprising: a light source emitting light at a modulation frequency andat substantially a single wavelength corresponding to a singleabsorption line at which water molecules absorb light at a substantiallygreater level than natural gas molecules; a detector configured todetect the intensity of light emitted from the light source at amultiple of the modulation frequency; and electronics coupled to thedetector for determining the level of water vapor in the natural gas. 2.A system as in claim 1, wherein the light source is a tunable diodelaser.
 3. A system as in claim 1, wherein the light source is colorcenter laser.
 4. A system as in claim 1, wherein the light source is aquantum cascade laser.
 5. A system as in claim 1, wherein the lightsource is a VCSEL.
 6. A system as in claim 1, wherein the detector is anInGaAs detector.
 7. A system as in claim 1, further comprising means forcalibrating the system relative to a known concentration of water vaporwithin the natural gas.
 8. A system as in claim 1, wherein the lightsource operates at a wavelength within the range of 1.8-1.9 μm.
 9. Asystem as in claim 8, wherein the light source operates at a wavelengthwithin the range of 1.877-1.901 μm.
 10. A system as in claim 1, whereinthe light source operates at a wavelength within the range of 2.7-2.8μm.
 11. A system as in claim 10, wherein the light source operates at awavelength within the range of 2.711-2.786 μm.
 12. A system as in claim1, wherein the light source operates at a wavelength within the range of920 to 960 nm.
 13. A method for determining trace amounts of level ofwater in natural gas comprising: generating modulated light at amodulation frequency and at substantially a single wavelengthcorresponding to a single absorption line at which water moleculesabsorb light at a substantially greater level than natural gasmolecules; passing the generated light through a sample of natural gas;detecting the light passed through the natural gas at a multiple of themodulation frequency; and determining a level of water within thenatural gas based on the level of detected light.
 14. A method as inclaim 13, wherein the generated light has a wavelength in the range1.877-1.901 μm.
 15. A method as in claim 13, wherein the generated lighthas a wavelength in the range 2.711-2.786 μm.
 16. A method as in claim13, wherein the generated light has a wavelength in the range 920-960nm.
 17. A system for detecting trace amounts of water vapor in naturalgas in a pipeline comprising: at least one optical gas sensor; a supplyline coupled to the pipeline and the optical gas sensor for supplyingnatural gas to the optical gas sensor; and whereas the optical gassensor comprises: a Herriott cell having two opposing mirrors; a lightsource emitting light modulated at a modulation frequency and atsubstantially a single wavelength corresponding to a single absorptionline at which water molecules absorb light at a substantially greaterlevel than natural gas molecules through the Herriott cell andconfigured to reflect off the mirrors to pass through the natural gas atleast two times; a detector configured to detect the intensity of lightemitted from the light source after the light reflects off the mirrorsat least two times at a frequency that is a multiple of the modulationfrequency; and electronics coupled to the detector to calculate thelevel of water vapor in the natural gas.
 18. A system as in claim 17,further comprising: at least one chemical gas sensor coupled to thesupply line.
 19. A system as in claim 18, wherein the at least onechemical sensor measures capacitance of a sensor element.
 20. A systemas in claim 18, wherein the at least one chemical sensor measures adielectric constant of a sensor element.